Abstract

An examination of cases of turnover in animal species on the Krakataus since 1883, particularly vertebrates, supports the findings of plant ecologists that very little, if any, turnover is stochastic. Successional, rather than equilibrium turnover is still occurring in all animal groups for which analyses can be made; for no group of animals is there evidence that an equilibrium of species number has been achieved, although for resident land birds there are indications that this is now imminent. Approach to equilibrium is not uniform; the colonization curves for resident land birds, reptiles, cockroaches and nymphalid and hesperiid Lepidoptera, as examples, have flattened markedly in the past 50 years, whereas numbers of species of land molluscs and many other insect groups are still increasing at a rate similar to that in the first half century since 1883. The period of the beginning of forest formation (1908-21) was the time when immigration reached a peak, and the period of canopy closure (1921-33) was the time of highest extinction rates. Successful colonists, over a range of animal groups, appear to be species with wide distributions and broad ecological tolerances. There is indirect evidence that successional processes have precluded colonization by several animal groups present in the mainland pool because of unavailability of their preferred habitat, and it is suggested that the effective available pool, as opposed to the theoretical one, changes in size and species complement as succession proceeds on the target islands. We believe that the brief open-habitat phase was too short for the establishment of several animal groups that were available in the pool. Animals of mature forest are, in general, absent as the archipelago’s forests are still relatively impoverished and early successional. Anak Krakatau, in general terms, offers an analogy with the early decades of colonization after 1883. The role of animals in the first stages of forest diversification from casuarina woodland has been monitored on this island, and an ash-lava aeolian ecosystem based on an allochthonous energy source was identified, which parallels similar systems on volcanic substrates in Sicily, the Canaries and, particularly, the island of Hawaii.

Footnotes

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